1. Field of Invention
This invention relates to modifications to conventional and existing reverberatory furnaces and to a new process for smelting copper concentrates in such furnaces for the production of matte--a semi-finished product--and of a furnace gas with high sulfur dioxide (SO.sub.2) content and for improving the thermal efficiency in such furnaces.
2. Description of the Prior Art
The smelting of copper concentrates has been performed in reverberatory furnaces for a very long time. Copper concentrates are charged in a reverberatory furnace either after roasting (calcine smelting) or as wet (5-10% moisture) concentrates (green charge smelting). The present discussion is restricted to green charge reverberatory smelting since the process of the invention may be applied to green (non-roasted) copper concentrates, and particularly because this invention describes a novel method of smelting copper concentrates in conventional and existing reverberatory furnaces which are modified accordingly.
A typical copper smelting reverberatory furnace is 100 to 120 ft. long, 28 to 36 ft. wide, and 13 to 15 ft. high, with a suspended roof composed of refractory bricks. At one end of the furnace powerful burners (natural gas, or fuel oil, or pulverized coal) supply highly heated products of combustion which melt the charge. The hot gases leave the furnace at the other end--opposite the burners--and flow through an uptake and through special waste-heat recovery boilers. As a rule, there are two waste heat boilers per furnace. Charging hoppers with water-cooled ducts through the roof and along the side walls of the furnace feed the process with wet concentrate mixed with the appropriate fluxes. The charge forms banks along the walls. The surface of the banks is exposed to the intense heat and smelts slowly. The charge melts into matte--a semi-finished product containing mainly cuprous sulfide (Cu.sub.2 S) and ferrous sulfide (FeS)--and slag, containing the fluxed oxidized and refractory impurities of the concentrate. Due to the difference in specific gravity, the matte settles under a layer of slag. The slag, with low content of copper, is skimmed close to the uptake of the furnace and is rejected. Matte is tapped from tap-holes (below the slag) into ladles and transported into special oxidizing furnaces, the converters. In the converters, matte is blown with air in the presence of silica flux. The iron and sulfur of the matte are oxidized to iron silicate slag and gaseous sulfur dioxide, and impure copper--called blister--is produced. The converter slag has a high copper content and is returned to the reverberatory furnace for cleaning. Reverberatory furnaces are equipped with one or two refractory launders for the recycling of the converter slag.
Only the unstable labile sulfur (from chalcopyrite, bornite, covellite, pyrite, etc.) is evolved by thermal decomposition and oxidized in the reverberatory furnaces, since the process is conducted under slightly oxidizing--almost neutral--conditions.
Reverberatory furnaces are thermally very inefficient with only 20-22% of the heat input consumed for smelting and the rest of the heat dissipated as sensible heat of voluminous combustion gaseous products and as extensive wall losses. A very significant consumption of fuel is required for green charge reverberatory smelting (6.0 to 9.0 million Btu's per ton of concentrate). In spite of the combustible nature of the sulfur and iron, that are contained in the charge, a typical reverberatory consumes 6 to 9 billion Btu's per day. The massive use of fossil fuels, in direct combustion within the furnace, is a huge waste of energy and creates voluminous combustion gases that dilute the gaseous products of smelting and, therefore, the furnace gas has a very low content of SO.sub.2 (1.0 to 2% at the uptake). This gas is further diluted by air infiltration in the boilers, flues and electrostatic percipitators and contains at the end 0.2 to 0.9% SO.sub.2. Very significant quantities of sulfur (up to 100 tons per furnace day) are emitted from copper smelting reverberatory furnaces. The very low content of SO.sub.2 in the reverberatory gas requires prohibitively high capital and operating cost for the control of the sulfur emission. The very high cost of controlling this sulfur emission and the very low thermal efficiency are the main drawbacks of conventional and existing reverberatory smelting.
Oxygen has been used in reverberatory furnaces mostly as oxygen enrichment of the combustion air. With the use of oxygen, the thermal efficiency of the furnace and its smelting rate have increased by 15 to 30%.
Oxy-fuel burners (combusting fuel with oxygen) firing through the roof on the banks of the charge have been also used in reverberatories and have improved the thermal efficiency and the production rate of the process. These improvements, nevertheless,--in spite of the modest increase in smelting rate--do not achieve furnace gas with high SO.sub.2 content.
There are new copper smelting processes--applied and proposed--which have significantly higher thermal efficiency than reverberatory furnaces and which produce gas with high content of SO.sub.2 amenable to pollution control. For instance, Morisaki et al U.S. Pat. No. 3,725,044 discloses a continuous method for the processing of sulfide ores by the use of a succession of distinct and uninterrupted unit furnaces which are in series to each other. Two or three furnaces are used. Although the Morisaki patent aims at achieving improved thermal efficiency as well as increasing the sulfur dioxide concentration in the waste gas, the method of the patent is described as "distinctly different" from conventional copper extraction methods including specifically reverberatory furnaces. The methods of the patent are of no possible benefit to achieving the same results in existing reverberatory furnaces which operate in a totally different manner and under relatively discontinuous batch-wise conditions. In the same manner, Worner U.S. Pat. Nos. 3,326,671 and 3,463,472 aim for producing metals directly from particulate ores and concentrates by the application of the method of the patents to a unique and integrated apparatus which, by the very nature of the method, precludes the use of this method for modifying existing reverberatory furnaces to obtain either improved thermal efficiency or an increased concentration of sulfur dioxide in the waste gas. Similarly, Holeczy et al U.S. Pat. No. 3,459,415 teaches the use of an "improved and specially designed apparatus" which directly produces converter copper.
All such new processes require in effect the complete abandonment of existing reverberatory furnaces and of most of the associated smelting equipment. Yet a number of reverberatory furnaces are in good operating condition and often better suited than the new processes for smelting contaminated copper concentrates. Replacement of operating reverberatory smelters by any of the new smelting processes is a very expensive proposition. The modification of existing reverberatory furnaces, as described by the present invention, can decrease very significantly their energy requirements and increase very substantially the SO.sub.2 content of the furnace gas.